1. Field of the Invention
This invention relates generally radar and range measurement systems and in particular to optical radar systems.
2. Description of Related Art
Conventional laser radar systems have several advantages over microwave radar systems. The primary advantage of this kind of radar is its very narrow beam width. This narrow beam width enables long-range performance with moderate transmit power. Conventional laser radar systems use techniques that are analogous to early rudimentary microwave systems. Conventional laser radar systems use short pulses to achieve fine range resolution, which limits range resolution as well as average power. Another drawback of conventional laser radar systems is the unsatisfactory way in which it deals with Doppler shifts.
Coherent optical detection is accomplished by mixing the incoming beam with a duplicate via a beam splitter. The Doppler shift appears in the beat frequency of the output of the beam splitter. However, coherent laser processing has two problems.
First, the Doppler frequencies for laser signals are many orders of magnitude higher than the Doppler frequencies for microwave signals. In applications of optical velocimetry to date initiated by the communications industry, all the emphasis has been on sensitivity issues rather than bandwidth issues (see, e.g., Kawai et al., xe2x80x9cUltrahigh-Sensitivity Self-Mixing Laser Doppler Velocimetry with Laser-Diode-Pumped Microchip LiNdP4O12 Lasersxe2x80x9d, IEEE Photonics Tech. Let., Col. 11, No. 6, June 1999, which is incorporated herein by reference in its entirety). There has been more interest in the industry for the detection of small numbers of photons than there has been to accurately measure the speed of a detected object. Consequently, there has been little research done of interest to radar researchers and the radar art.
A second problem with laser radar is the phase noise of laser devices. Phase noise cancellation has been the subject of some recent research, again in the communications industry. However, it is still a formidable problem. Examples of research in this area include articles by Taylor, R., and Forrest, S., xe2x80x9cOptically Coherent Direct Modulated FM Analog Link with Phase Noise Cancelling Circuitxe2x80x9d, J. Lightwave Tech., Vol. 17, No. 4, April 1999; Gross, R., Olshansky, R., and Schmidt, M., xe2x80x9cCoherent FM-SCM System Using DFB Lasers and a Phase Noise Cancellation Circuitxe2x80x9d, IEEE Photonics Letters, Vol. 2, No. 1, January 1990; and Kwong, W., Prucnal, P., and Teich, M., xe2x80x9cCoherent Subcarrier Fiber-Optic Communication Systems with Phase-noise Cancellationxe2x80x9d, IEEE Transactions on Comm., Vol. 42, No. 6, June 1994. Each of the above-identified references is incorporated herein by reference in its entirety.
Fortunately, with the advent of very high-speed modulators, some aspects of modern microwave radar are now applicable to laser radar. A comparison between modern microwave radar, and modern laser radar is provided in the following table:
Table 1 is a comparison between microwave radar and infrared (IR) laser radar. This table shows clearly the difference in duty cycle and Doppler shifts as measured by the two systems. Several of the design implications as a result of these characteristics on laser radar systems are discussed in the following section. Those skilled in the art will appreciate that the peak power of radar is the average power divided by the duty cycle. Thus, for microwave radar, the peak power is greater than the average power by a factor of 5. For the laser the difference is a factor of 105, thereby placing a very high power requirement on the laser. The contrast is even starker in relation to Doppler processing. Because of the poor short-term stability of a laser coupled with the 5+ order of magnitude, the difference between laser Doppler and microwave Doppler effectively precludes lasers from tasks requiring range-rate information. Since the vast majority of radar applications either require Doppler processing or would greatly benefit from it, the implications of this drawback significantly undermines the use of lasers in radar systems.
Accordingly, it is an object of the present invention to provide an improvement in laser radar systems.
It is yet another object of the invention to provide a laser radar, that has the benefits of microwave radar, along with the benefits of laser radar.
The foregoing and other objects are achieved by a method of implementing a laser radar, the method comprising: amplitude modulating a laser beam with a source signal; transmitting a signal comprising light packets at a repetition rate of the source signal, wherein the transmitted signal is generated from amplitude modulating the laser beam; receiving a reflected signal that is a reflection of the transmitted signal; and determining a Doppler frequency shift from signal properties of the reflected signal.
Further, the foregoing and other objects are achieved by a laser radar system comprising: a transmitter that transmits a signal, wherein the transmitted signal is formed by an optical signal that is amplitude modulated with a source signal, the transmitted signal comprising light packets at a repetition rate of the source signal; a receiver that receives a reflected signal that is a reflection of the transmitted signal; and a Doppler processor that determines the Doppler frequency shift of the source signal from signals derived from the reflected signal.
Further scope of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood, however, that the detailed description and specific embodiments, while disclosing the preferred embodiments of the invention, are provided by way of illustration only inasmuch as various changes and modifications coming within the spirit and scope of the invention will become apparent to those skilled in the art from the detailed description which follows.